Measurement of the electrical potential, or electrical field strength, on the skin of a person by means of electrocardiographic sensors forms the basis of many medical diagnostic methods. In this way, for example, an electrocardiogram (ECG) may be recorded or the heart rate may be determined from the measured electrical potentials.
In conventional measurement methods for measuring the electrical potential on the skin, the latter is acquired by electrodes which are in direct electrical contact with the surface of the skin. An electrically conductive connection is thus established between the skin, on the one hand, and the electrode, on the other hand. In this case, however, it often proves difficult to ensure a sufficiently good electrical contact between the electrode and the skin, and therefore the body of the person being examined (the subject). Furthermore, the use of such diagnostic methods is also increasingly being provided in application fields in which direct access to the skin of the subject is not available, for example in vehicle applications for monitoring body functions and/or vital parameters of vehicle passengers on seats or bunks.
For example, U.S. Pat. No. 7,684,854 B2 discloses a sensor for contactless electrocardiographic measurement on a person. The person may in this case be on a stool, in a bed or on a vehicle seat. The electrocardiogram can be recorded from the body of the person wearing clothing without direct contact with the skin. The sensor comprises a flat electrically conductive electrode which comprises a measurement surface facing toward the person and a connection surface which faces away from the person, lies opposite the measurement surface and is electrically connected to a preamplifier. The electrode and the preamplifier of the sensor are enclosed by shielding.
Another contactless sensor for recording an electrocardiogram of a person is disclosed by EP 2 532 306 A1. The sensor comprises an electrically conductive electrode and a detection device, which is electrically connected to the electrode and is configured in order to amplify the signals received by the electrode. The sensor is intended to be arranged in a vehicle seat and to determine particular physiological parameters of a driver sitting on the vehicle seat.
DE 20 2012 001 096 U1 discloses capacitive sensors for capacitive recording of vital parameters of a driver of a vehicle. To this end, the sensors are fitted in or on the backrest of the seat of the vehicle. In particular, according to one embodiment it is proposed to arrange the sensors in or on the backrest of the seat while being distributed in two rows separated by a distance corresponding to the width of the spinal column of the driver. In each row, the sensors, with an area of from 16 to 36 cm2, are arranged at equal distances of from 1 to 5 cm from one another. In another embodiment, instead of the two separate sensor rows with sensors distributed over the entire height of the seat at a distance of 1-5 cm, two membrane sensors with a width of from 4 to 10 cm are arranged over the entire seat height with a separation corresponding to the spinal column.
Furthermore, DE 10 2008 049 112 A1 discloses a capacitive textile electrode for measuring body functions and/or vital parameters of persons for vehicle applications, for example in a seat or a bunk, which electrode has a multilayer structure. This comprises two textile layers, each of which has an electrically conductive electrode region, a further textile layer being provided in order to establish a distance between the other two textile layers.
In general, contactless electrocardiographic measurement is distinguished in that there may, for example, be clothing between the skin of the subject and the electrode of the sensor. The signal quality of a measurement signal registered in this way by the electrode may, however, be substantially influenced by various factors. Such factors may include, for example, the vibrations to which the sensor is exposed at the time of the measurement, the contact pressure which exists between the subject and the electrode at the time of the measurement, the microclimate between the electrode and the skin of the subject, the materials of the clothing, as well as the electrostatic charge in the vicinity of the electrode.